1. Field of the Invention
The present invention relates to a color monitor utilizing ambient light illumination and, more particularly, to a color analyzer simultaneously responsive to light intensity at multiple wavelength regions emitted by or reflected from a specimen illuminated by existing ambient light for instantaneously determining relative ratios of the wavelength regions.
2. Description of the Prior Art
In the field of color measurement, it has been the general practice to mount a specimen within a chamber having a carefully controlled light source. The relative spectral reflectance at various wavelengths is then determined by either a scanning spectrophotometer or a multiple array of photodetector-spectral filter modules.
In the case of a scanning spectrophotometer, light energy from wavelength regions of greatest interest is integrated and then subjected to various computational procedures so as to obtain numerical values related to the relative reflectance characteristics at the wavelength regions of interest. Since this method usually requires several minutes to scan the spectrum, sample or light instabilities create major errors. In addition, the complexity of the computations makes this type of system very expensive.
The multiple detector-filter instruments usually employ three detectors having spectral response characteristics closely matching those of the CIE Standard Observer. The Standard Observer consists of values recommended by the Commission Internationale de l'Eclairage (CIE) giving the response of each of three assumed photochemical processes in the eye to light of each of the wavelengths of the visible spectrum. These response functions are designated x, y, and z, x = red, y = green, z = blue, and are known as the tristimulus values.
The CIE Standard Observer spectral response characteristics cover three very broad regions between 400 and 750 nanometers. These broad wavelength characteristics make it very difficult to detect small color differences between two samples. Furthermore, the accuracy is sharply degraded by metamerism, a phenomenon whereby two samples with markedly different spectral compositions can have the same appearance, as well as tristimulus values, with one particular lighting situation but appear considerably different under another type of illumination.
In either the spectrophotometer or multiple detector-filter instruments, the specimen generally must have uniform color, be of a relatively large size, and be capable of being held flat. This greatly limits the usefulness of these instruments, especially for process control applications.